The present invention relates to roto-molding plastic containers and more specifically to the roto-molding of multi-layered plastic containers.
Roto-molding is well-known in the art and U.S. Pat. No. 3,885,016 generally describes the roto-molding process. Briefly roto-molding is a process used to fabricate plastic vessels, such as cylindrical tanks, rectangular tanks and other hollow forms and the like. In roto-molding a predetermined amount of powdered thermoplastic material, such as polyethylene is loaded into a mold. The mold is closed and placed into an oven where it is simultaneously rotated about two axes. As the thermoplastic material melts and the mold is rotated, the plastic coats the interior of the mold to form a seamless homogeneous structure with a uniform wall thickness. The mold is then moved out of the furnace and into a cooling chamber to solidify the plastic, after which the molded plastic container is removed.
One typical roto-molding arrangement has three horizontal arms extending outward from a central journal to form a generally Y-shaped arrangement. One roto-mold is then mounted at the end of each arm. This provides a three-station manufacturing process wherein one station has an oven, the second a cooling chamber and the third a loading position. With such an installation, the three arms are simply rotated about the central journal to move one of the molds progressively from a loading station where the mold is charged with powdered resin, to the furnace where the mold is heated and simultaneously rotated about two axes to form the container, to the cooling chamber where a water spray is directed at the mold to speed the cooling cycle. The mold is then moved back to the load station where the completed container is stripped from the mold and another charge of resin is introduced into the mold.
It is well-known that some thermoplastic resins have better corrosion and abrasion resistance than others. However, such resins are usually more expensive, so that containers roto-molded entirely of the more chemical or abrasion resistant resin is considerably more expensive than the same container molded from a resin which is less corrosion or abrasion resistant. A good compromise between these two situations is to provide a container having a laminated structure wherein the less expensive, less resistant material forms a substrate with the more expensive, more resistant material being the topmost layer.
The present invention provides a method and apparatus for roto-molding multi-layered containers by introducing separate charges of powdered resin into the mold while the mold is in the oven. This permits the formation of a multi-layered product without the need for removing the mold from the oven and disrupting the heating cycle.